Project Summary Transport of molecules and ions across the plasma membrane is essential to cell growth and proliferation, but despite its vital role in biology, very little is known about the molecular mechanisms that underlie transport cycles. One of the most important classes of cellular nutrients is amino acids, which are the building blocks of proteins and also important constituents of diverse cellular processes including energy generation, neurotransmission, and nitrogen metabolism. Consistent with their fundamental biological role, several families of proteins have evolved to transport amino acids, with one of the largest of these families being the APC (Amino acid, Polyamine, Organocation) transporters. APC transporters are found in all kingdoms of life and are involved in several disease conditions in humans, including cancer and asthma. At the present time, there is no atomic resolution structure of an APC transporter, greatly hindering our understanding of their architecture, translocation mechanism, and substrate specificity. This application, proposes to determine the structure of an APC transporter using x-ray crystallography. Furthermore, the homologue used in the structural determination will be characterized functionally and used to test structure- based mechanisms of transporter function. Together, these studies will yield unique insight into the molecular method of translocation across the membrane and should facilitate the design of new molecules that may have therapeutic potential. Relevance to Public Health The accumulation of nutrients from the environment is essential for all life, but very little is understood about this process. Perturbation of nutrient uptake is involved in numerous human disease states, including asthma and cancer. This study aims to elucidate the fundamental mechanism involved in nutrient acquisition and application of this knowledge makes possible the design of novel therapeutics. [unreadable] [unreadable] [unreadable]